Mechanisms governing the growth of self-assembled quantum dots.

Description

We produced self-assembled quantum dot (QD) samples of InAs on GaAs by molecular beam epitaxy (MBE). With these, we explored growth effects as a function of InAs coverage for three arsenic pressures, and as a function of arsenic pressure at a specific InAs coverage. During growth, the samples were studied using reflection high energy electron diffraction (RHEED). These RHEED measurements were compared to low energy electron diffraction (LEED) measurements. To perform this ex-situ LEED characterisation, some samples were covered with an amorphous arsenic cap. This cap was thermally evaporated producing a clean, non-oxidised surface that was studied using LEED. We obtained non-ambiguous identification of the GaAs (001) surface reconstructions as well as timing information for the 2D to 3D transition during the growth of InAs on GaAs. Post growth characterisation of two sets of self-assembled QD samples, twelve samples in all, revealed the following: As a function of increasing the arsenic pressure used in QD growth, the photoluminescence (PL) of capped QDs is first redshifted at low arsenic pressures, and then blueshifted at high arsenic pressures. Scanning electron microscopy and atomic force microscopy of uncapped QDs show that as the arsenic pressure increases, the QD density increases while the average QD width and height decrease monotonically; these trends are consistent with the shift in PL for the high arsenic pressure samples, but are inconsistent with the shift in PL for the low pressure samples. This leads us to proposing a mechanism by which QDs may be modified as they are overgrown with capping material. We discuss the effects of adjusting the arsenic pressure on the formation of QDs and the mechanism by which QDs may be modified during capping.